Confinement and stability of plasmas in a field-reversed configuration

Abstract

Experiments have been conducted on the LSX device where plasmas confined in a field-reversed magnetic geometry have exhibited record energy, particle, and configuration lifetimes. The scaling from previous smaller devices showed a very positive confinement scaling with J, the number of ion gyroradii inside the field-reversed configuration. These plasmas were observed to have gross stability to global loworder modes such as the internal tilt. The growth of tilt instabilities was not observed during the equilibrium decay of plasmas up to s~8.PACS numbers: 52.55.Pi A new experimental device (LSX) has been constructed to explore the confinement and stability of magnetically confined plasma known as a field-reversed configuration (FRC). An FRC has a compact toroidal confinement geometry where induced toroidal diamagnetic currents generate the confining poloidal field [1]. Since the toroidal field is typically small inside the FRC, the configuration has an intrinsically high plasma p (the magnetic field vanishes at the plasma core). This feature together with the simple cylindrical geometry, the natural diverter nature of the external magnetic field, and the ease of translation [2] give the FRC unique advantages as a fusion reactor-particularly for advanced fuels [3].From a theoretical point of view, the key issue has been the gross stability of the configuration. For several years it has been known that the FRC equilibrium, as a result of the unfavorable curvature of the closed field lines (see Fig. 1), is unstable in the magnetohydrodynamic (MHD) limit. In particular, the FRC is unstable to the internal tilt instability [4]. This global mode consists of a rotation of the FRC orthogonal to the symmetry axis with only a small distortion to the separatrix shape, and thus cannot be suppressed by the application of an external magnetic field. The mode has a growth rate that would annihilate the configuration on the shortest possible MHD time FIG. 1. Schematic of the LSX experiment. Equilibrium flux contours are shown from a 2D numerical calculation for an FRC in LSX. scale, the time for an Alfven wave to propagate the half length of the FRC.Stability for many Alfven transit times has been observed in several FRC experiments [1,5]. The FRCs formed in these experiments, however, had a strong kinetic particle component. This component can be characterized by the parameter s, the average number of ion gyroradii between the null and separatrix of the FRC, and s was typically 2 or less. It was shown that for s < 2, kinetic effects significantly increased the MHD tilt growth time [6] to as long as the FRC configuration lifetimes obtained at that time. Subsequent experiments on the TRX device, however, indicated stability to the tilt at moderate s (2 to 4) for many kinetically increased tilt growth times [5]. The confinement did not improve with increasing s as had been observed in previous low-j, kinetic-regime experiments. It was not determined what limited confinement; however, the equilibria produced in these large-.? experim...

The Pulsed High Density Experiment: Concept, Design, and Initial Results

“…A major concern for the steady-state low density FRC reactor is the stability to tilt, as well as other instabilities, at the high flux required for a reactor. In current and past experiments, the ratio of the separatrix radius to the average ion gyroradius (commonly referred to as s) was 4 or less [2]. This parameter has been used to characterize the kinetic contribution to stability, with significant reduction in mode growth rates for s < 3.…”

“…The requirement on the FRC closed poloidal flux is reduced to what has already been achieved so that the FRC should remain MHD stable through burn. Most importantly, the FRC has already demonstrated the confinement scaling with size and density required for fusion at high density [2]. The FRC has demonstrated the confinement scaling with size and density required for fusion at a confining pressure well below the yield strength of materials allowing for solenoidal confining magnets.…”

Macron Formed Liner Compression as a Practical Method for Enabling Magneto-Inertial Fusion

“…It is important to know whether this compression speed is consistent with the FRC formation, translation and equilibrium lifetimes. FRC lifetime scaling obtained from previous experiments stated in terms of externally measured parameters was found to be [12,15]:…”

“…Of all fusion reactor embodiments, only the FRC has the linear geometry high plasma , and closed field confinement required for magnetic fusion at high energy density. Most importantly, the FRC has already demonstrated the confinement scaling with size and density required to assure sufficient lifetime to survive the compression timescale required for MIF over a wide range of conditions [12,13]. Thus the target plasma for the macron liner experiments to be employed here will be the FRC.…”

Inductive sustainment of oblate field-reversed configurations with the assistance of magnetic diffusion, shaping, and finite-Larmor radius stabilization

“…63 Measurements in LSX, however, were unable to detect any sign of the tilt. 64,65 Furthermore, decaying prolate FRCs are often able to last for many Alfven transit times. 44 These prolate FRCs are typically very kinetic ͑i.e., the ions have Larmor orbits that are comparable to the system size͒, which should contribute a stabilizing effect.…”